Communications system and method

ABSTRACT

A communications system ( 1 ) includes a plurality of nodes ( 2 ). Each node ( 2 ) has receiving means for receiving via an antenna ( 13 ) a signal transmitted by wireless transmitting means; transmitting means for wireless transmission of a signal via an antenna ( 13 ); and, means for determining if a signal received by said node ( 2 ) includes information for another node ( 2 ) and causing a signal including said information to be transmitted by said transmitting means to another node ( 2 ) if said received signal includes information for another node ( 2 ). Each node ( 2 ) has one or more substantially unidirectional point-to-point wireless transmission links ( 3 ). At least some of the nodes ( 2 ) have plural substantially unidirectional point-to-point wireless transmission links ( 3 ) which share at least one of a receiver ( 10 ) of the receiving means and a transmitter ( 11 ) of the transmitting means. Each of the links ( 3 ) is to one other node ( 2 ) only. At least some of the nodes ( 2 ) are the origination and termination point of user traffic.

[0001] This is a Division of U.S. application Ser. No. 09/331,219 filedon Jun. 17, 1999, which is a U.S. national phase of PCT/GB97/03472 filedon Dec. 18, 1997, the entirety of which is hereby incorporated herein byreference.

[0002] The present invention relates to a communications system andmethod.

[0003] There is an increasing demand for high bandwidth communicationssystems which can carry data at rates which are significantly higherthan those which are presently available to business or residentialusers. Systems which would benefit from very high data transfer ratesinclude video-on-demand, video conferencing and video “telephony”,business and home Internet access, local area networks (LAN)interconnects, virtual private networks, teleworking, on-line games,high definition television, and many other applications demanding highinformation transfer rates.

[0004] In a conventional telephone communications system, the systemoperator's main switched trunk network is connected to an access networkwhich connects the trunk network to a subscriber's individual telephonehandset or private branch exchange (PBX). The access network is oftenknown as the “local loop”.

[0005] The vast majority of local loop networks in the United Kingdomand many other countries are based on wires which are either buried inthe ground or are suspended overhead from poles. The wire extends fromthe regional access switch to the subscriber and is essentiallydedicated to one subscriber and carries signals for no-one else.

[0006] Copper wire has conventionally been used primarily because of itsrelative low cost. However, copper wire can only carry data at a rate ofabout 2,400 to 9,600 bits per second (bps) without data compression.With more sophisticated techniques, this limit has been increased toabout 57,000 bps. However, this is extremely slow when compared with therate required for real-time video, which is in the region of 2 to 9million bps (Mbps).

[0007] Some UK operators are now offering digital access services usingthe integrated services digital network (ISDN) system. However, the datatransfer rate is still only about 64,000 to 128,000 bps with ISDN orISDN2 and wired technology is still used. More recently, wired systemssuch as HDSL (high speed digital subscriber line) and ADSL (asymmetricdigital subscriber line) can deliver up to 2,000,000 bps (2 Mbps).However, as these are still wired systems, there is a very substantialstart-up cost for any such system in that the operator must incur thesignificant cost of digging up roads, pavements, etc. to lay the cablesor wires to a large number of subscribers before the system can beginoperating. Indeed, the operator must take a large financial risk whensetting up a new wired system in that the operator must lay a very largenumber of cables or wires before potential customers have committedthemselves to the system so that the operator can offer a system whichis already functional. This is obviously a significant risk,particularly where new technology is involved and the level of customertake-up of the system is unknown at the time the operator installs theinfrastructure for the system.

[0008] Similarly, in a conventional, point-to-multipoint (broadcast)cellular system, each subscriber unit deals only with informationintended for that subscriber.

[0009] Both the standard telephone system and cellular system mentionedabove require some form of central station sending information to andreceiving information from outlying or peripheral subscriber stations.

[0010] A wireless system is very much cheaper to install as nomechanical digging or laying of cables or wires is required. User sitescan be installed and de-installed very quickly. Thus, radiocommunications systems have many attractive features in the area oflarge-scale system deployment. However, it is a feature of radio systemswhen a large bandwidth (data transfer rate) is required that, as thebandwidth which can be given to each user increases, it is necessary forthe bandwidth of the radio signals to be similarly increased.Furthermore, the frequencies which can be used for radio transmissionare closely regulated and it is a fact that only at microwavefrequencies (i.e. in the gigahertz (GHz) region) or higher are suchlarge bandwidths now available as the lower radio frequencies havealready been allocated.

[0011] The problem with microwave or higher frequencies is that theseradio frequencies are increasingly attenuated or completely blocked byobstructions such as buildings, vehicles, trees, etc. Such obstructionsdo not significantly attenuate signals in the megahertz (MHz) band butbecomes a serious problem in the gigahertz (GHz) band. Thus,conventional wisdom has been that microwave or higher frequencies aredifficult to use in a public access network which provides communicationwith a large number of distributed users.

[0012] The spectral efficiency of any wireless communications system isextremely important as there are many demands on radio bandwidth. As amatter of practice, the regulatory and licensing authorities are onlyable to license relatively narrow regions of the radio spectrum. Acellular system, which uses point-to-multipoint broadcasts, places highdemands on the radio spectrum in order to provide users with asatisfactory bandwidth and is therefore not very efficient spectrally.

[0013] The use of repeaters or relays to pass on data from one stationto another is well known in many applications. However, in each case,such repeaters broadcast signals, in a point-to-multipoint manner, andare therefore similar to a cellular approach and suffer from acorresponding lack of spectral efficiency.

[0014] According to a first aspect of the present invention, there isprovided a communications system, the system comprises a plurality ofnodes. Each node has receiving means for receiving via an antenna asignal transmitted by wireless transmitting means; transmitting meansfor wireless transmission of a signal via an antenna; and, means fordetermining if a signal received by said node includes information foranother node and causing a signal including said information to betransmitted by said transmitting means to another node if said receivedsignal includes information for another node. Each node has one or moresubstantially unidirectional point-to-point wireless transmission links.At least some of the nodes have plural substantially unidirectionalpoint-to-point wireless transmission links which share at least one of areceiver of the receiving means and a transmitter of the transmittingmeans. Each of said links is to one other node only. At least some ofthe nodes are the origination and termination point of user traffic.

[0015] Wireless transmission is used to provide communication with eachnode. In practice, each node is likely to be equipment associated with auser of or subscriber to the system. Each node is preferably stationaryor fixed. The nodes operate in a peer-to-peer manner, which is incontrast to the central-master/peripheral-slave manner of say a cellularbroadcast system. In the present invention, information is typicallytransferred in a series of “hops” from node to node around the systembetween a source node and a destination node. In the preferredembodiment, the nodes are logically connected to each other by pluralpoint-to-point links between each linked pair of nodes and can beregarded as providing an interconnected “web” covering a geographicalarea and providing a non-cellular network. The links are substantiallyunidirectional, i.e. signals are not broadcast but are instead directedto a particular node with signals being capable of being passed in bothdirections along the link.

[0016] It will be appreciated that some prior art systems have nodeswhich can communicate with each other with the nodes acting as simplerepeaters. However, the individual transmissions in such prior artsystems are often omnidirectional or use wide-angled transmissionsectors and so such systems are still fundamentally cellular instructure. Such prior art systems thus tend to use point-to-multipointtransmissions, using a master/slave or central/peripheral architecture.In the preferred embodiment of the present invention, the nodes areconnected in a peer-to-peer manner, with point-to-point links, in aninterconnected mesh. In the present invention, many links across thesystem or network may be “active”, that is carrying signals, at the sametime so that plural pairs of linked nodes may be communicating with eachother substantially simultaneously. In the preferred embodiment, foreach node, only one link is “active” at any one time and the link isactive in only one direction at a time (i.e. a node is eithertransmitting only or receiving only on that link). In other words, if anode is transmitting or receiving on one of its links, it will not bereceiving or transmitting on any of its other links. This greatlyincreases spectral efficiency compared to a cellular system or othersystems using broadcast transmissions from a node. This configurationalso helps to keep down the cost of the individual nodes as each nodeonly requires one transmitter and one receiver.

[0017] Each node of the invention may be autonomous with respect to, forexample, the transmission of signals to other nodes and need not bereliant on control signals from some central controller or any othernode. “Calls” between nodes can be effectively asynchronous and a callbetween a pair of nodes can start and finish effectively at any time,substantially independently of the state of any other call.

[0018] In an example of the invention, each node is a subscriber unitwhich can be mounted on or near a subscriber's house. In addition,further nodes may be strategically placed in other suitable placesaccording to the requirements of the operator. Thus, it is not necessaryto provide metal (e.g. copper) wire, fibre optic or other fixed “hard”links to each user, which saves the very high costs of digging up roads,laying fixed cables, etc. This means that the entry cost for a providerof the system can be relatively very low. A small system providingaccess for say a hundred or a thousand users can be set up very cheaplyand additional users can be added later as demand grows.

[0019] In contrast to conventional point-to-multipoint broadcast radiosystems, the present invention does not require a central transmitterwith an extremely high bandwidth to service the subscribers' datademands. In fact, except for possible interfacing to a trunk network, nohigh capital cost, high-profile, high-complexity sites are required forair-side interfacing, switching and transmission. These functions can bedelocalised over the whole network in the system described herein.Moreover, the present invention does not require the large and unsightlyradio masts/towers which are typical of cellular systems.

[0020] Nodes, as well as carrying traffic intended for other nodes, canalso be the origination and termination point of users traffic. This hasbenefits for expansion of the network because, in principle, traffic canbe injected and extracted from any node in the network, unlike cellularsystems where a high-profile location (such as a hill top) has to bechosen for this purpose for example.

[0021] One or more nodes may be associated with plural users of orsubscribers to the system. For example, a small business may have onenode to which their internal LAN (local area network) is connectedwhereby all of the LAN users can access the communications system. Anode with a bandwidth of say 2 Mbps could support up to 200 users eachrequiring a bandwidth of 9,600 bps.

[0022] Each node is used to pass on or “route” those signals whichinclude information intended for other nodes in the system. If a nodeshould fail in the system of the present invention, there is a loss ofservice only for the subscriber (if any) associated with that node andinformation for other nodes can be routed through nodes other than thefailed node in the preferred embodiment.

[0023] Information is passed as necessary in a series of “hops” from onenode to another via a preferably predetermined route until theinformation reaches its destination node.

[0024] The nodes are preferably linked so as to form plural transmissionpath loops thereby to provide plural choices of path for thetransmission of a signal between at least some of the nodes. Each looppreferably consists of an even number of links. This allows for propersynchronisation of transmission and reception between nodes.

[0025] For each node that has plural links to other nodes, each of saidplural links to another node is preferably associated with a time slot.Each link for each node may be associated with a distinct time slot.Thus, where TDM (time division multiplexing) is used, no node has morethan one link having the same time slot number in the TDM framestructure.

[0026] The allocation of time slots to the links may be varied such thata link may selectively be associated with more than one time slot. Thisallows the effective bandwidth supported by a particular link to beincreased, perhaps temporarily, as required by a user associated with aparticular node for example.

[0027] Each node preferably has a direct line-of-sight link with atleast one other node such that each node can transmit a signal toanother node in line-of-sight with said each node. It will be understoodthat line-of-sight means that the path between two nodes connected by aline-of-sight link is entirely or substantially unobstructed such thatthe path is transparent or substantially transparent to the frequencyused.

[0028] “Information” in a signal may be for example software, whetherfor the operation of the node itself or for use by a subscriberassociated with the node or otherwise, voice telephony data, video data,or telecommunications traffic generally.

[0029] Preferably, a signal including said information is transmitted bya node to another node if and only if a signal received at said nodeincludes information for another node.

[0030] The number of nodes is preferably less than the number of links.This serves to ensure that there can be several distinct paths betweenany two nodes. Also, because the traffic equations areunder-constrained, the traffic flowing on a link is not only a functionof the subscriber injected/removed traffic, but also a function of thetraffic on other links. This leads to a large number of possible trafficconfigurations for any given subscriber traffic. This means that (i) thepoint-to-point capacity of the network is increased relative to chainand tree topologies, (ii) it allows scope for network managementstrategies to alter traffic flows in parts of the network to preventcongestion without, in principle, adversely affecting the trafficcarrying-capacity of the network as a whole, and (iii) the spectralefficiency of the system can be greatly improved over conventionalcellular radio techniques.

[0031] Each node is preferably arranged to be in a transmission mode fora time period which alternates with a time period for a reception mode.

[0032] Other duplex techniques, such as Frequency Division Duplex (FDD),may be used.

[0033] Because each node is concerned with switching as well as thetransmission of information traffic, the whole system can effectivelybehave as a distributed switch. This means that conventional accessswitches (i.e. exchanges), which represent significant capitalexpenditure, can be eliminated.

[0034] Many topologies for connecting the nodes are possible. Possibletopologies include a fully interconnected topology, in which each nodeis directly connected to each other node; a linear chain topology, inwhich each node is connected to two other nodes in a chain; a treetopology, in which each node is connected to a predetermined number ofother nodes such that there are no loops in the topological structure; alattice topology, in which each node is connected to up to apredetermined number of nearest neighbours; and, a hypercube-typetopology in which each node is linked to n other nodes. Non-regulartopologies, with for example a random interconnection of nodes and/or ahigh degree of interconnectivity, are also possible and have manydesirable properties. For example, a non-regular topology (like certainregular topologies) may provide a large number of possible routes forinformation to pass across the system or web. Combinations of topologiesare also possible. For example, a hypercube structure of dimension ncould service clusters of n fully interconnected n-valent nodes. Astructure close to a perfect hypercube could alternatively be used forexample.

[0035] It will be appreciated that in most areas where the system isdeployed, the location of the nodes is dictated by the subscriberlocations and that lines of sight between the nodes depends on the localgeography. In such situations, it is unlikely that a prechosen networktopology can be mapped onto the available lines of sight. A morepragmatic approach is to build up the network from the available linesof sight, carrying out the process with a view to creating a networkwith the desired traffic-bearing characteristics. Computer modelling hasbeen carried out and it has been shown that it is possible to fulfil therequirements and preferred features of the network without having aregular form. The modelling indicates that structures worked up from theactual physical connectivity can perform well with regard totraffic-bearing properties.

[0036] Preferably, at least one node is arranged not to transmit to anyother node information in a signal received by said at least one nodewhen that information is addressed to said at least one node. Mostpreferably, all nodes operate in this manner.

[0037] Each node preferably has addressing means for adding toinformation in a received signal the address of a node to which a signalincluding said information is to be routed when said information is foranother node. Thus, each node can easily pass on information intendedfor other nodes.

[0038] The addressing means may include means for determining the routeof information through the system and adding an appropriate address tothe information accordingly.

[0039] The nodes may have means for determining the route of informationthrough the system as a whole.

[0040] Alternatively, the route of information through the system may bedetermined centrally by a central system controller. Thus, there may beprovided a central system controller for determining the route ofinformation through the system. The system may be used for passingcontrol signals from the central system controller to each node.

[0041] At least one node may have means for determining if a receivedsignal includes information for said at least one node and processingmeans for processing information in a signal addressed to said at leastone node. All nodes may operate in this manner.

[0042] The transmitting means of the nodes preferably transmit signalsat a frequency of at least about 1 GHz. A frequency greater than 2.4 GHzor 4 GHz may be used. Indeed, a frequency of 40 GHz, 60 GHz or even 200GHz may be used. Beyond radio frequencies, other yet higher frequenciessuch as of the order of 100,000 GHz (infra-red) could be used. (The UKWireless Telegraphy Act 1949 defines the upper frequency limit for theradio spectrum as 3×10¹² Hz.) The receiving means are arranged toreceive signals at the frequencies transmitted by the transmittingmeans. It will be understood that, at least from a practical technicalpoint of view, a greater bandwidth is more easily obtained if a higherfrequency is used with suitable modulation.

[0043] The link between two nodes may be arranged to use simultaneouslytwo or more frequency channels. This reduces the bandwidth load on aparticular frequency channel.

[0044] The receiving and transmitting means may be arranged to transmitand detect circularly polarised radiation. The transmitting meanspreferably includes a highly directional transmitter antenna. Thereceiving means preferably includes a highly directional receiverantenna. Each of these preferred features helps to prevent interferencebetween nodes and also helps to mitigate the effects of multipathing.

[0045] All nodes may be substantially identical. This simplifies theimplementation of the present invention and helps to keep down costs.

[0046] The system can effectively be a self-contained network. On theother hand, by way of example, the system may be an access networkconnected to a conventional trunk network for providing access tosubscribers or to other networks. A further node may be connected by adata connection to one of the nodes of the system and arranged totransfer a signal to or receive a signal from the trunk network or both.

[0047] One or more data storage servers can be connected to or providedat suitable nodes in the system. Various types of data can be stored onsuch data storage servers. For example, for so-called network computing,a user's software applications can be stored at a data storage serverremote from that subscriber's node. The user accesses those applicationsthrough the system of the present invention. The applications can beeasily updated by the software =producer and can be used by pluralsubscribers who perhaps pay the software producer on a time-usage basis.The data stored on the data storage servers could be data for videossuch as films (movies). This would not only provide a distributedvideo-on-demand service, but, in addition, from the system operator'spoint of view, would allow video material to be distributed to theembedded servers using the same system possibly in a broadcast mode. Ineither case, frequently requested material migrates from main systemlibraries out to points in the system where it is required. Thismoderates the bandwidth requirements both for the video servers and foroperator's libraries.

[0048] Plural systems, each as described above, can be provided witheach system being connected to at least one other system. The connectionbetween such systems can be a radio connection, a wired connection suchas a fibre optic link, or any other suitable means.

[0049] At least one link of a node may be arranged to use a firsttransmission frequency and at least one other link of said node may bearranged to use a second transmission frequency. This can be used tohelp prevent interference between nodes.

[0050] In an embodiment, some of the nodes are allocated to subscribersand some of the nodes are not allocated to subscribers, at least some ofsaid non-allocated nodes being solely for carrying information trafficbetween subscriber nodes.

[0051] According to a second aspect of the present invention, there isprovided a method of communications across a network of nodes. Each nodehas one or more substantially unidirectional point-to-point wirelesstransmission links. At least some of the nodes have plural substantiallyunidirectional point-to-point wireless transmission links which share atleast one of a receiver and a transmitter. Each of said links is to oneother node only. The method comprises the steps of: (A) originating userdata at one of the nodes; (B) transmitting a signal including said userdata from said node to another node along a substantially unidirectionalpoint-to-point wireless transmission link between said nodes; (C)receiving said signal at said other node; (D) determining in said othernode if the signal received by said other node includes user data for afurther node and transmitting a signal including said user data fromsaid other node to a further node along a substantially unidirectionalpoint-to-point wireless transmission link between said nodes if saidreceived signal includes user data for a further node; and, (E)repeating steps (B) to (D) until said user data reaches its destinationnode.

[0052] Preferably, for each node that has plural links to other nodes,each of said plural links to another node is associated with a timeslot, and each transmission step on a link of said one node occursduring a distinct time slot and each receiving step on a link of saidother node occurs during a distinct time slot. The allocation of timeslots to the links may be varied such that a link is selectivelyassociated with more than one time slot.

[0053] Each node preferably adds to user data in a received signal theaddress of a node to which a signal including said user data is to berouted when said user data is for another node.

[0054] Each node may have addressing means, the addressing meansdetermining the route of the user data through the system and adding anappropriate address to the user data accordingly. Alternatively, acentral system controller determines the route of user data through thesystem.

[0055] The method preferably comprises the step of each nodetransmitting a signal including said user data to another node if andonly if a signal received at said node includes user data for anothernode.

[0056] The method preferably includes the steps of determining in atleast one node if a received signal includes user data for said at leastone node and processing the user data in a signal addressed to said atleast one node.

[0057] Preferably, the signals are transmitted at frequencies greaterthan about 1 GHz.

[0058] There may be at least two possible paths for transfer of databetween a source node and a destination node. In such a case, the methodmay comprise the step of transmitting a copy of said data on each ofsaid at least two paths. Alternatively, the method in such a case maycomprise the steps of: transmitting from the source node a part only ofsaid data on each of said at least two paths and reconstructing the datafrom said transmitted parts of said data in the destination node. Thiscan increase the effective bandwidth of transmissions and allowsredundancy to be achieved.

[0059] According to another aspect of the present invention, there isprovided a telecommunications switching apparatus, comprising acommunications system as described above.

[0060] Embodiments of the present invention will now be described by wayof example with reference to the accompanying drawings, in which:

[0061]FIG. 1 is a schematic representation of a first example of asystem according to the present invention;

[0062]FIG. 2 is a schematic representation of a second example of asystem according to the present invention;

[0063]FIGS. 3 and 4 are schematic representations of further examples ofsystems according to the present invention;

[0064]FIG. 5 is a schematic representation of a further example of asystem according to the present invention;

[0065] FIGS. 6 to 9 are schematic representations of differenttopologies for the system of the invention;

[0066]FIG. 10 is a schematic illustration of a node showing the radiocomponents;

[0067]FIG. 11 is a schematic representation of a time slot structure ofa node timing frame;

[0068]FIGS. 12A to 12C show matrices for explaining the allocation oftime slots to links;

[0069]FIG. 13 is a representation of a portion of an example of a systemaccording to the present invention showing synchronism of time slots;

[0070]FIG. 14 is a representation of a portion of a further example of asystem according to the present invention showing possible interferencebetween nodes;

[0071]FIG. 15 is a schematic representation of a simplified system forexplaining the addressing of signals within a hypercube topology;

[0072]FIGS. 16 and 17 show examples of routing algorithms; and,

[0073]FIGS. 18 and 19 show examples of connection of systems accordingto the present invention to a trunk network.

[0074] In an arbitrary network having a total of N nodes and a total ofE interconnections or links, at each node the traffic flowing into itminus the traffic flowing out of it must be the net traffic introducedby the subscriber associated with that node (neglecting any buffering).If T_(ij) represents the traffic flowing from node i to node j, andB_(i) the user traffic at node i, then the following must be true at anyinstant of time:

Σ_(i=0,N) T _(ij) =B _(j), and T _(ij) =−T _(ji), and T _(jj)=0 for j=0to N

[0075] (Traffic Constraint Equations)

[0076] Treating the link traffic T_(ji) as unknowns, and the usertraffic as known, there are N+E constraint equations and 2E unknowns,where the exact topology of the network dictates how N and E arerelated. There are two network topology classes of interest for presentpurposes, namely topologies for which N≧E and topologies for which N<E.

[0077] The first type of network topology with N≧E implies that thetraffic equations above are completely constrained, i.e. the trafficflowing in any link is completely determined by the known subscribertraffic injected/removed from the network. Networks of this type can beconstructed by adding only one new link every time a new node is added.Regular forms of such networks are for example one-dimensional chainsand trees (where E=N−1), the topologies encountered in conventionalaccess networks. Another property of these networks is that there isonly one possible route between any two nodes (without traversing anylink twice): there are no loops. Network systems having topologies withN=E may be single chain loops, possibly combined with linear chains andtrees; for these systems, there is a maximum of two paths between anytwo nodes.

[0078] The other class of network topology, where the number of possiblelinks exceeds the number of subscriber nodes (N<E), is of more interestfor the purposes of the present invention. This is for two main reasons.First, there can be several distinct paths between any two nodes.Second, because the traffic equations are under-constrained, the trafficflowing on a link is not only a function of the subscriberinjected/removed traffic, but also a function of the traffic on otherlinks. This leads to a large number of possible traffic configurationsfor any given subscriber traffic. These are highly desirable propertiesbecause (i) the point-to-point capacity of the network is increasedrelative to chain and tree topologies, (ii) it allows scope for networkmanagement strategies to alter traffic flows in parts of the network toprevent congestion without, in principle, adversely affecting thetraffic carrying-capacity of the network as a whole, and (iii) as willbe shown later, the spectral efficiency of the system can be greatlyimproved over conventional cellular radio techniques.

[0079] To achieve the above desirable properties, the network ispreferably constructed such that multiple paths between arbitrary nodesare possible, i.e. the network contains transmission path loops.

[0080] Even in networks in which N<E, connections to trunk networks formpotential bottlenecks where diverse traffic streams are forced through asingle link. This implies that the capacity and location of trunknetwork connections will need to be planned with care. Conventionalaccess networks are dimensioned on the 80/20 rule-of-thumb, that is, byjudicious choice of region, approximately 80% of the traffic generatedby subscribers is confined to that region, with only 20% requiringaccess to the trunk network and this approach can be applied in thepresent invention.

[0081] The capacity of the network or “web” depends on how the nodes areactually connected. Consider the example of a network 1 shown in FIG. 1in which each node 2 has a link 3 with its nearest neighbours only. (Itwill be understood that the lines which represent the links 3 betweennodes 2 in the drawings are only schematic and show which nodes 2 areconnected to which other nodes 2 via point-to-point line-of-sightwireless transmissions.) The links 3 between nodes 2 will typically becarrying information not just for the neighbouring nodes but also fornodes further down the path. The amount of bandwidth required for agiven bandwidth ‘delivered’ will depend on the proportion of thebandwidth to be passed on by a node, compared with that being deliveredto the node. This in turn depends on the average number of ‘hops’ that apiece of information has to make to get to its destination. The numberof ‘hops’ taken to get from one node to the next depends on exactly howthe nodes are connected. In the example of FIG. 1, if information is tobe sent between A and 0, a route such as ABCDEJO has to be used,requiring a lot of hops. However, if the network were as shown in FIG.2, the route could be ANO, requiring many fewer hops.

[0082] Thus, it is desirable to find ways of connecting nodes thatminimise the number of hops and maximise the number of nodes connected,while at the same time keeping the number of links per node to areasonable number. This latter point is important since, trivially, afully interconnected web in which all nodes linked to all others isclearly the best in that number of hops required to transmit between anytwo nodes is only one, but the number of links per node is equal to thenumber of nodes and so becomes large very quickly.

[0083] One way of looking at the number of hops (H) problem is toconsider the access area serviced (A) to be randomly populated with Nsubscribers. On average, the width of the area will be ≈{square root}Aand the mean distance between subscribers will be ≈{square root}(A/N).Thus, the number of hops across the region will be H≈{square root}N,assuming most nearest neighbours are interconnected. In networks of theorder of 10⁶ subscribers, this implies 1,000 hops to traverse thenetwork. Given that each hop introduces a finite delay (td) into thetraffic streams retransmitted, it is essential to minimise the productof t_(d) and H. A total end-to-end delay of <50 ms is a useful target.For nearest neighbour connectivity, this means that the individual nodedelay must be <50 μs. It is clear that nearest neighbour interconnectionschemes will probably give rise to unacceptable traversal delays wherethe number of nodes is relatively large.

[0084] A mix of nearest neighbour and more remote point-to-point(line-of-sight) connections may therefore be appropriate. In this way,the number of hops across the network is related more to itsline-of-sight properties than its subscriber density. For example, ifthe mean line-of-sight distance for a particular network is L, thenH≈{square root}A/L, and so if L>{square root}(A/N), the number of hopsacross the network will be significantly reduced.

[0085] A simple method of ensuring that a system or web 1 of the presentinvention does not have a nearest neighbour-only topology will now bedescribed with reference to FIGS. 3 and 4. As shown in FIGS. 3 and 4,part or all of a web 1 is notionally divided into M (arbitrary)geographical regions of roughly similar populations where M is themaximum valence of a node, i.e. M is the maximum number of links 3 whichcan be supported by a node 2. In the example shown, M is eight. Inpractice, any such geographical division will have to take account oflines-of-sight available. (Note that other nodes 2 in the web 1 andtheir connections have been omitted from FIGS. 3 and 4 for clarity.)

[0086] Looking at region A in FIG. 3, it can be seen that the node q inregion A has been connected by a link 3 to other nodes 2 such that nomore than one connection has been made to a node 2 lying in the sameregion. Connecting all the nodes 2 in this way will clearly ensure thatwebs 1 having nearest neighbour connections only are avoided. Strongerforms of this method are possible. For example, connections may be madeas above, but which exclude any connection to a node 2 (such as node p)in the same region. In practice, the exact form of strategy adopted willdepend on the geography and the ultimate range of a node 2. Anothervariant of the above scheme, which could be used where node range wasrestricted, would be to connect only to neighbouring regions, withinrange, as shown in FIG. 4.

[0087] It is important to know what bandwidth is required on each of thelinks in order to set up links of bandwidth B between randomly chosenpairs of nodes until all nodes are connected. Now, to answer thisquestion fully is complex because it depends on the required trafficcharacteristics and the permissible routing algorithms, and wouldrequire the general solution of the constraint equations above. However,the following gives a simple calculation to find the required bandwidthb of a link to sustain traffic in a web or network, where each node inthe web is sinking and sourcing bandwidth B. For a network which cancope with arbitrary, symmetric subscriber interconnections, ideally:

b≈B  (1)

[0088] i.e. the required link bandwidth should be independent of thenumber of subscribers in the network and be of the order of the offeredtraffic at each node.

[0089] Assume that the network is a non-nearest neighbour web, and, as aworst case, that the data a node is sinking/sourcing is being exchangedwith the most distant node in the network. Since the number of nodes ina web is N, and if each node is sending data to one other node, thenthere are N connections active. For this web, assume that there are nhops on average between a node and the most distant node from it.

[0090] The subscriber traffic therefore requires nBN units of bandwidthfrom the network. Now, if the web has E links each of which can carry 2bunits of data (b in each direction), the network therefore has 2bE unitsof bandwidth available. Thus if routing issues are ignored, then2bE=nBN.

[0091] Thus, each link carries traffic of bandwidth nBN/2E. If b≈B, thennBN/2E≈B, or nN/2E≈1, so that:

n≈2E/N  (2)

[0092] Thus the link bandwidth constraint (1) implies a constraint onthe mean number of hops across the web (2) in terms of the number ofnodes and links comprising the web in that from the point of view ofdesirable bandwidth properties, the quantity 2E/N should be of the orderof the mean number of hops across the web.

[0093] In a practical system, n should be as small as possible forreal-time services, as large n means larger transit delays. However,since E/N is related to the number of loops possible in the web, thisshould be as large as possible to exploit the desirable propertiesoutlined above. In practice, a compromise value must be found.

[0094] To examine traffic congestion issues, a symmetry argumenttogether with a simple conceptual routing algorithm for the web may beused. One simple routing algorithm specifies that traffic going from onenode to a second node will be split evenly at each intermediary nodeover each of the links leading further towards the destination. Thiscould be done by, for example, a simple statistical multiplexing scheme.Thus for the first half of the journey the traffic is smeared out overthe web, and for the second half the traffic concentrates towards thedestination node. If only a single connection were active, then withthis algorithm the traffic density would be higher around the twoterminal nodes and sparser between them. When all the connections areactive, the contributions to traffic density will tend to average out,depending on the web symmetry. If there is a high degree of symmetrythroughout the web, the number of traffic “hot-spots” will be minimisedand the routing will tend not to block. Thus, to increase theload-balancing properties of the network, it is desirable that thetopology be as symmetric as possible.

[0095] It is instructive to consider what the above traffic propertiesmean in a radio context. If it were possible to create webs of nodeswith the above properties, N nodes could be interconnected with links ofbandwidth B using only a radio spectrum of B Hz (using the simplifyingassumption of one bit per hertz). In fact, for practical reasons, thiscannot be easily achieved (and this is discussed in detail later), butthis property is extremely important as it shows that this structure isfundamentally very much more spectrally efficient than the cellulararchitecture, as will be discussed further below.

[0096] A simple practical example of a network or web system 1 accordingto the present invention is shown in FIG. 5. In the example shown, thereare sixteen subscribers or users, each of which is associated with anetwork node 2. The system 1 is connected via interconnect trunks 4which connect specified nodes 2 to a trunk network 5. Each node 2 has aradio transceiver unit which is able to transmit and receive high radiofrequency signals, for example at least 1 gigahertz (GHz) or 2.4 GHz or4 GHz or even up to or greater than 40 GHz. The transceiver unit of eachnode 2 is in direct line-of-sight contact with four other similar unitsat other respective nodes 2 by direct line-of-sight links 3. Again itwill be understood that the lines which represent the links 3 betweennodes 2 in FIG. 5 are only schematic and show which nodes 2 areconnected in a point-to-point manner to which other nodes 2 via wirelesstransmissions. It can be seen from FIG. 5 how the nodes 2 of a system orweb 1 according to the present invention can communicate with each othervia other nodes 2 if necessary to avoid buildings 6 or otherobstructions which otherwise block the direct line-of-sight connectionbetween particular nodes 2. It should be noted that each node 2 in thisexample of the system 1 be connected to the same number n of other nodesin a hypercube topology. This results in efficient use of the system 1.However, it is possible for some nodes in the system 1 to be connectedto less than n other nodes in a less-than-perfect hypercube.

[0097] As mentioned above and as will be further explained below, amessage from any one particular node 2 to any other particular node 2will usually traverse several links 3 between several nodes 2 in aseries of “hops” across the system 1. Each passage of a signal through anode 2 produces a delay in transfer of the signal. The delay might beonly a millisecond or so, but if there were a very large number ofnodes, this delay could rapidly build into significant fractions of asecond. Such relatively long delays would not be generally acceptable ininteractive services such as voice traffic, video conferencing, etc.Thus, it is highly desirable to minimise the maximum number of hopsrequired by a signal in transferring across the system 1. For example,the hypercube structure provides an efficient way of connecting manyusers with a small number of maximum hops required to transfer a signalbetween a source node and a destination node.

[0098] Furthermore, each link 3 has a certain fixed information carryingcapacity, determined in large part in practice by the bandwidth of thecarrier signal used to transmit information between nodes 2. Each link 3carries information data intended for a node connected to the link 3 andalso “transit” data intended for other nodes. Indeed, each link 3carries approximately n times the amount of transit data for eachinformation data carried by the link. Thus, it is generally better tohave a relatively small number of links 3 between nodes 2 (i.e. a smalldimension topology) because this increases the bandwidth available toeach message as fewer messages in total have to be carried by each link3.

[0099] In a system having a hypercube-type topology, if each node islinked to n other nodes, the maximum number of nodes in such a system,which is equivalent to the maximum number N of users of the system, is2^(n) where there is just one subscriber per node 2. The maximum numberof hops required to transmit information from any node to any other isn. The total number of links E=n.2^((n−l))=(N/2)log₂N. There are n!possible topologically equivalent routes for information to cross thesystem, meaning that good service can be maintained for the vastmajority of users even if one or more individual nodes fails for somereason as other routes for messages to cross the system can be found.For example, to service a region of 65,536 users using a hypercubetopology, where, for simplicity, there is one user per node, n=16. Inother words, for a system for 65,536 users, each user node needs to beconnected to 16 other user nodes and a maximum of 16 hops are requiredto transmit information from any one node to any other node in thesystem.

[0100] Topologies having a high degree of node interconnectivity supportmany possible equivalent routes through the system 1, each having arelatively low number of hops. Node complexity, in terms of the numberof links 3 required by each node 2, scales only very slowly with thesize of the system 1 in a topology such as a hypercube topology. Theratio of user bandwidth to the resultant link bandwidth is low, possiblyless than unity because of the multiple routing possibilities. Nodes 2can be low cost because of the modest bandwidth requirements. The nodes2 can be identical, leading to low installation costs and ease ofoperation, management and maintenance.

[0101] The factors which will decide the optimum topology to be usedinclude message traffic patterns, geography of the land in which thesystem is implemented, user location density, and system application(e.g. video-on-demand or Internet web-browsing).

[0102] One alternative topology is a fully interconnected topology shownby way of example in FIG. 6. Each node 2 is connected to each other node2 and thus for an N node network, each node 2 must support (N−1)external links 3 to other nodes 2. The total number of links 3 istherefore N(N−1)/2. This topology is most suited to a relatively smallnumber of nodes 2, for example where N is less than 10. Adding nodes 2to such a system 1 means that all existing nodes 2 must be modified tointerconnect to any new node 2. The main advantage of such as system 1is that only one hop is required to transfer a message from any one node2 to any other node 2. Thus, a fully interconnected topology is mostsuited for connecting a small fixed number of nodes 2.

[0103] Another alternative topology is a linear chain topology shown byway of example in FIG. 7. Each node 2 is connected to two others. In asystem 1 of N nodes 2, there are thus N links 3 and information willrequire N/2 hops to cross the system 1. Because all message traffic isconcentrated onto the chain of links 3, each link 3 must be of highbandwidth (approximately N/2 times the bandwidth required by each node2), which may limit the number of nodes which can be connected in such atopology. A main advantage of such a topology is the comparativesimplicity of the nodes 2 which each have only two external links 3.

[0104] A further example of a suitable topology is a tree topology asshown by way of example in FIG. 8. In a homogeneous tree topology, everynode 2 is connected to a fixed number of other nodes 2 in such a waythat there are no “loops”, i.e. there are no paths which can be followedwhich pass through the same node 2 more than once. For a tree with nodes2 connected to J other “lower” nodes 2, and having L levels, the numberof nodes 2 is the geometric series:${\sum\limits_{k = 0}^{L}J^{k}} = \frac{1 - J^{L + 1}}{1 - J}$

[0105] which for large J tends to J^(L). A disadvantage of this topologyis that at each hop away from a node 2, the nodes 2 must service J timesthe peak bandwidth of the node connection, implying greatly increasedbandwidth requirements on descending the tree. Another disadvantage isthat the nodes 2 differ between levels as they must function differentlymeaning that a system provider must deploy and manage different nodesfor each level. However, an advantage is that at most two hops arerequired to transmit a message from any node 2 to any other node 2 inthe same level (for example, the lowest level in FIG. 8).

[0106] An inhomogeneous tree topology relaxes the requirement for thenumber of connected lower nodes 2 to be fixed, though in other respectsis similar to the homogeneous tree topology described above.

[0107] A yet further example of a suitable topology for connecting thenodes 2 is a lattice topology shown by way of example in FIG. 9. Nodes 2are connected in an arbitrary manner to up to a fixed number n ofnearest nodes 2. In a grid structure a portion of which is shown in FIG.9, where n=4 and serving say N=10,000 nodes 2, a message may require{square root}N=100 hops to cross the system 1, which may lead tounacceptable traversal delays. Also, the bandwidth requirements of eachlink 3 may be high as it will be approximately ({square root}N)/2 timesthe bandwidth required by each user.

[0108] It will be appreciated that whatever topology is selected for thesystem, it must be flattened onto the effectively two dimensionalgeography of a geographical region, inevitably leading to a requirementfor some links 3 to be longer than others. With present technology, highfrequency transmitters transmitting say 40 GHz frequencies only have arange of about 500 m to 2 km or perhaps at best up to about 4 or 5 km.There is therefore a problem in providing links 3 between nodes 2 whichare more than about 2 km apart. This can be overcome by limiting asystem to a relatively small number of nodes 2, say 1,024 nodes 2. Sucha system 1 can then be connected to other similar systems 1 of the sameor similar size using a large antenna and radio link, a fibre opticlink, etc. Systems 1 having different topologies can be connected to oneanother.

[0109] The network 1 may effectively be a mixture of topologies.

[0110] In the preferred embodiment, there are multiple links per nodeorientated in arbitrary directions. This could be achieved with multipleradio systems per node. However, when compared with a typical cellularsystem which would only have one radio system per subscriber, this islikely to make the nodes significantly more expensive than theircellular equivalent. This is especially true when the radios areoperating in the high GHz where this element of the system is likely tobe a significant part of the node cost.

[0111] To achieve 360 degree angular coverage at a node, it is possibleto use one or more antennas which are steerable either electrically orphysically and which can point in any azimuthal direction, or an arrayof fixed antennas each pointing in a different direction such that anyparticular direction is accessible from one of the antennas.

[0112] The exact number M of antennas must be chosen to allow completeangular coverage without adversely affecting link gain. Note, M may begreater than n, the maximum active links per node. However, rather thanproviding M pairs of transceivers at each node, each pair beingcontinuously connected to a single antenna, for cost reasons it ispreferred to use only one transceiver per node and make use of timedivision multiplexing (TDM) and time division duplex (TDD) techniques toconnect the transceiver to an antenna. A node therefore has only onetransceiver pair which must be able to use all M antennas. TDM can beused to time-share the antennas with the transceiver. TDD can be used toalternate the receive/transmit operation of the node radio so that it isnever receiving and transmitting simultaneously. Frequency divisionmultiplexing or code-division multiplexing could be used as alternativesto TDM. Frequency division duplex could be used as an alternative toTDD. Other alternative schemes may be possible.

[0113] The basic structure of the radio frequency parts of a node 2 isshown in FIG. 10. A receiver 10 and transmitter 11 are alternativelyconnected to an M-way switch 12 which conducts radio-frequency (RF)power from and to the antennas 13.

[0114] A simple scheme of scheduling the connection of antennas 13 isshown in the time slot structure in FIG. 11 for the case M=8. Time isdivided equally into “frames” 20 and each frame 20 is divided equallyinto a transmit phase 21 and a receive phase 22. The transmit andreceive phases 21,22 are themselves divided into equal time slots 23.Each one of these time slots 23 is used for one link 3 from a node 2.Thus, the node 2 transmits in one time slot 23 on one link 3 then thenext time slot 23 on the next link 3 and so on, followed by receiving inone time slot 23 on one link 3 and the next time slot 23 on the nextlink 3 and so on. Each receive time slot 23 of each node is arranged tobe long enough to ensure that there is sufficient time for a signaltransmitted from other nodes 2 to travel to the node 2 in question andalso to be received in full at the node 2 in question, particularly toensure that the data packet and any guard bands are received.

[0115] In-turn sequencing is not the only possible way of addressingantennas 13. The total bandwidth available at a node 2 can bepartitioned by allocating more or fewer time slots 23 to an antenna 13,within a receive or transmit half frame 21,22. This is illustrated inthe matrices in FIG. 12. The columns of the matrices represent the eightantennas 13 on an example node 2, and the rows eight possiblereceive/transmit time slots 23. A ‘1’ in a cell indicates which antenna13 is active during which time slot 23. A ‘−’ in a cell indicates noactivity of an antenna 13 in a particular time slot 23. The number of‘1’ s must not exceed the total number of time slots 23 available.

[0116] In FIG. 12A, each antenna has a time slot, so each link can carry1 unit of bandwidth. In FIG. 12B antenna A0 has two time slotsallocated, and hence can carry two units of bandwidth. Antennas A1, A2,and A7 each have one time slot allocated, and antenna A4 has three timeslots allocated. Antennas A3, A5, and A6 have no allocated time slotsand hence are idle. In FIG. 12C all the time slots have been given toantenna A4. This means that link associated with antenna A4 can carryeight units of bandwidth whilst all the others are idle.

[0117] It may be noted that whilst TDM/TDD is used to divide up timebetween links 3, this does not imply that the time a link 3 spendsactive is also divided into time slots. As each link 3 connects only twonodes 2, there is no need for a further time-division structure, formultiple access purposes, on a link 3 for the purposes of the presentinvention.

[0118] Considering now the need for synchronisation of transmission andreception by the nodes 2, if any one of the nodes 2 is transmitting thenall the nodes 2 to which it is transmitting must be receiving. This isonly possible with certain web topologies. Many topologies satisfy thistransmit/receive phasing if all transmission path loops in the web havean even number of sides.

[0119] Not only must communicating nodes 2 be transmitting or receivingin synchronism but they must agree on the time slot number that they areusing. Referring to FIG. 13, nodes A and B must both be using the sametime slot for the link 3 between them, say time slot 1 transmit for Aand time slot 1 receive for B. Similarly, A and C must use the same timeslot for the link between them, say time slot 2, etc. However, each node2 may only use each time slot once. In the preferred embodiment, thisrequirement is met exactly throughout the network. Thus, each link 3 inthe network 1 is assigned a time slot number such that no node 2 hasmore than one link of the same time slot number. In addition, it isdesirable to minimise the total number of time slots required. If themaximum numbers of links per node is M, it is clear that at least M timeslots are needed. For any network topology with loops having an evennumber of sides, if M is the maximum node valence of the network, then Mtime slots can be consistently allocated in this network.

[0120] It will be appreciated that different groups of nodes 2 may becommunicating with each other at any one time. In other words, differenttransmission paths in the system 1 may be active and carrying traffic atany one time.

[0121] Reference is now made to FIG. 14 in which part of a web 1 isshown. Using the above described transmit/receive synchronisation andtime slot allocation rules, nodes ABCDEF will not interfere with eachother. However, there may be a problem with nodes G and H. This isbecause the link between nodes D and C uses time slot 2. Now, the radiosignal for this link will not stop at C, but will continue on and may bedetected by the receiver in node H which will also be receiving duringthat time slot using an antenna pointing in a similar direction. Intheory it may be possible to design network topologies which somehowavoid this situation, but given the complexity of real-world subscriberpositioning, this is likely to be infeasible in practice. The system inpractice should therefore be arranged so that, even though thegeometrical arrangement is as shown, the fact that D's signals aredetectable at H does not cause interference to the signals received at Hfrom E.

[0122] This can be achieved by using a set of frequency channels andassigning one of these to each link in the network in such a way thatall potentially interfering links are on different channels. The set ofchannels should be as small as necessary. This requirement for a minimumnumber of frequency channels is related to the beam width of the nodeantenna. For large widths, the area of the interference zone PDQ in FIG.14 is also large and hence there is a greater likelihood of nodes suchas G and H lying in it. Similarly, for small beam widths, the zone areais small, thus containing fewer nodes.

[0123] In the example shown, this would mean that link DC is on adifferent frequency channel to link GH. Allocating frequency channels isa complex task. Some system modelling has been done to investigate thisissue with the outcome that the frequency reuse factor is similar to thecellular case, i.e. somewhere between 6 and 10.

[0124] The implication of this on design of the nodes 2 is that theradio system must be frequency-agile, re-tuning to a differentpre-allocated channel on each time slot.

[0125] As with all communication systems, individual links 3 are liableto suffer from interference and damage. Very short timescale problemsare handled by standard means, including Forward Error Correction andre-transmissions. On occasion, a link 3 may suffer problems thateffectively make it useless. However, with a web according to thepreferred embodiment, there will always be a large number of equivalentroutes between any two nodes 2 so the loss of some links 3 can becountered by re-routing the connection.

[0126] Link loss occurs on several timescales. In the medium term, atemporary loss for some seconds or minutes may be caused by largevehicles moving nearby, or perhaps a plume of smoke from a fire. Thenetwork will cope with these by re-routing traffic to avoid the problemareas until the link recovers. On a longer timescale, a link 3 may belost because line of sight is being permanently obstructed. This may becaused by new building or tree growth. These losses should be handled ata network planning level. As a background activity, the network mayconstantly monitor all available lines of sight, (i.e. links 3 betweennodes),including those which are not currently being used for subscribertraffic. On a timescale of hours and days, or even minutes or seconds,the network can be automatically reconfigured to use different subsetsof the available lines of sight to optimise operational parameters.

[0127] Some subscribers may have very stringent requirements for linkavailability and require high integrity links so that theircommunications are not vulnerable to single point failure. When carryingsuch traffic, multiple paths (m) through the network may be used. Twomethods of operation are possible. In the first, each path carries aduplicate of the subscriber data, so that the receiving node 2 mayaccept data from any active path. This uses up m times the basicsubscriber bandwidth (B) for the connection, but is simple to implement.In the second, each path carries part of the subscriber data (with someadditional parity information) so that the receiving node 2 canreconstruct the data from any m−1 paths received and the parityinformation. This uses in total only αB units of bandwidth (α=parityinformation overhead>1). The second example of method of operation canbe extended to protect against multiple path failures but is morecomplex than the first example of method of operation.

[0128] The availability of multiple paths is an inherent property of thepreferred embodiment of a web network 1 of the present invention. Bycomparison, provision of multiple physical paths in a cable or wirebased network is enormously expensive.

[0129] In above description, one time slot 23 is used to support all ofthe bandwidth on a link 3. This maximises the raw data transfer rate;however, it is always important to maintain spectral efficiency.

[0130] A general calculation of the spectral efficiency of a network inaccordance with the present invention compared to conventional cellularapproaches is not easy to calculate as much depends on the exactimplementation. However, a cellular approach requires approximately:

[0131] α.N.B_(subs).F_(cell) units of bandwidth, where:

[0132] α is the maximum fraction of active subscribers

[0133] N is the number of subscribers

[0134] B_(subs) is the bandwidth required by a subscriber

[0135] F_(cell) is the cellular frequency reuse factor,

[0136] and assuming a modulation technique giving one bit/Hz.

[0137] The present invention requires approximately n.B_(link).F_(web)units of bandwidth, where:

[0138] n is the maximum number of links/time slots on a node

[0139] B_(link) is the bandwidth of a link

[0140] F_(web) is the frequency reuse factor needed to minimiseinterference in the present invention,

[0141] again assuming a modulation technique giving one bit/Hz.

[0142] F_(cell) is typically in the range 6 to 10 and computer modellinghas shown F_(web) to be much the same. Computer modelling has beencarried out for a number of scenarios and a reasonable set of parametersis that n=8 and that B_(link) is equal to B_(subs).

[0143] This gives the efficiency of the web approach to the cellularapproach as:

(α.N)/n

[0144] For a cell covering 1000 users and a peak active load of 20% (atypical estimate for video-on-demand services), the relative efficiencyis 25 fold. This is extremely important as there are many demands onradio bandwidth and as a matter of practice the regulatory and licensingauthorities are only able to license relatively narrow regions of theradio spectrum. The present invention places much lower demands on theradio spectrum than a cellular system providing a comparable userbandwidth.

[0145] A simple example of a routing protocol will now be described. Thesystem 1 is well suited to the use of asynchronous transfer mode (ATM)technology which can support connection oriented (circuit switched) orconnectionless (packet switched) traffic modes by the transfer of 53byte information “cells”.

[0146] In a hypercube topology network with n connections at each node2, each out-going connection can be labelled with an index (0 . . .n−1). A path through the network system 1 can then be defined by a listof such indices. As will be understood from the above, the maximumlength of this list will be n entries.

[0147] In general, an information packet can be defined to be of typeMessage which has two components:

[0148] information cell payload (cell), and

[0149] the routing address (L)

[0150] The routing address is the absolute address of a node 2 in thenetwork system 1. Each node 2 will have access to its own address (my_Lin the code discussed below). To see how addressing works in such asystem 1, consider the addressing of points on a simple unit 3-cubeshown in FIG. 15. Each node 2 has a labelled set of channels which canbe thought of as Cartesian axes, in this case X, Y and Z. Thus each node2 has an X-channel, a Y-channel and a Z-channel.

[0151] The address (L) of a node 2 in a 3-cube geometry is one of theeight 3-vectors: (0,0,0), (1,0,0), . . . (1,1,1). A move through thecube by one hop along a link 3 (i.e. traversal of an edge) isrepresented by the following relationship between the initial (L1) andfinal (L2) address:

|L1−L2|=1

[0152] Thus, a “forward” move is defined by L1−L2=1 and a “backward”move by L1−L2=−1.

[0153] The routing algorithm shown in FIG. 16 replicated in each node 2of the system 1 will in principle ensure correct cell routing.

[0154] The function of the handleReturnedMessage function is to takeappropriate action with a returned message. This strategy will depend onthe type of data service supported. It could be one of the following:

[0155] 1. Return the message to sender, i.e. propagate the message allthe way back to the originator. This should signal to the originatorthat there is congestion and that it should pause sending informationfor a period of time.

[0156] 2. Store the message for a period of time, then attempt toforward it to its destination as before.

[0157] 3. Forward the message forcing a different route to be taken, forexample, by choosing an output channel which has low congestion.

[0158] 4. Discard the message, assuming that a higher-level data-linkprotocol will detect the loss and cause the originator to re-transmitthe message.

[0159] The procedure SendPacket (msg, next_node) conceptually sendsMessage msg to the outgoing link 3 with index next_node. The procedureProcessCell (cell) is responsible for consuming the information celllocally and making it available to the user.

[0160] The decideNextChannel function has a functionality which isnetwork topology specific. For the case of a hypercube topology, anexample of this is set out in FIG. 17, where ActiveChannels is thenumber of currently configured channels on a node 2 (which may vary foreach node 2 in the system), and MaximumChannelUtilisation is the valueat and above which the outgoing channel may be considered to be at fullcapacity and can therefore accept no further traffic.

[0161] Where the instantaneous utilisation of an output channel is ameasure of the traffic loading of that channel over an immediatelyprevious period of time. Such a measure of traffic loading might be one,or a combination of, the following factors:

[0162] 1. The number of currently allocated communications circuits onthe link

[0163] 2. The amount of data sent on the link.

[0164] In addition, the ChannelUtilisation function may be used tocontrol non-existent links as in the case of a partially completehypercube. In this case, the link utilisation could be set permanentlyto MaximumChannelUtilisation.

[0165] A continuously operating function of a node would be themonitoring of this loading and allow the routing software to obtain avalue related to the current loading for a given link. This is what thefunction ChannelUtilisation (channel) does.

[0166] The procedure MapWeightedChannelToBestChannel translates theinput weighted channel index into a real output channel for the node.The simplest, non-trivial case would be where output channels aredenoted by integer values, 0 to 7 for example, and the mapping of thereal weighted channel number to this is simply a rounding operation. Forexample, weighted channel value 6.7152 is mapped to channel index 7.

[0167] The performance of the system 1 has been primarily described sofar in terms of its ability to move data within a cluster of nodes 2.However, for many types of service it is required to connect into atrunk network 5, as indicated in FIG. 5. For example, in a network ofsay 250 users used primarily for 5 Mbps video-on-demand (VOD) servicewith a loading of say 0.3 Erlang per user, the total bandwidth requiredfrom the trunk network is 375 Mbps, assuming that no source materialmigration takes place. As the maximum input rate to a node will be sayabout 40 Mbps (assuming eight links 3 of maximum 5 Mbps each per node2), this 375 Mbit/s of traffic will need to be groomed onto the trunknetwork at at least ten locations. This can be done in two ways.

[0168] The first alternative is to connect the subscriber interface of anode 2′ at each of the “input” locations to a suitable interface on thetrunk network 5 (e.g. DS3, STM0, 1) as shown in FIG. 18. The nodes 2′ atthe input locations can be connected by an optical fibre 4 to the fibrebackbone of the trunk network 5 for example. These input locations canbe chosen for network deployment convenience rather than by subscriberlocation. This is much easier than running fibre to cellular type basestations where the positions of the base stations are dictated by thecellular structure.

[0169] The second alternative is to configure a set of nodes 2″ so thatall time slots 23 are used on one link. This provides several point topoint connections with exactly the right bandwidth (40 Mbps) forconnection into a node 2. The specially configured nodes 2″ can beconnected by a suitable data connection 6 to a normal subscriber node 2at the same location. It should be noted that these specially configurednodes 2″ can use exactly the same hardware as the normal subscribernodes 2. However, the specially configured nodes 2″ could each use ahigh gain, long range movable antenna if required. Such antennas couldbe directed at a cluster 7 of suitably configured nodes 8, located at asingle trunk access point 9 as shown in FIG. 19.

[0170] A problem with many radio communications systems is multipathing.This can occur when a receiver receives a main signal received directlyfrom a transmitter but also receives signals from the transmitter whichhave been reflected from buildings or moving vehicles, for example. Thereflected signal is delayed relative to the main signal, which can leadto cancellation of the main signal if the reflected signal is an oddnumber of half wavelengths lagging in phase. With medium wavetransmissions, where wavelengths of several hundreds of meters are used,cancellation is not much of a problem; the user can usually find aposition for the receiver where cancellation because of reflections frombuildings does not occur or, where cancellation occurs because of asignal reflected from a moving vehicle, the cancellation only occursbriefly and the vehicle moves away, thereby removing the problem.However, at higher frequencies, where wavelengths might be severalmillimeters, objects moving past a receiver can cause frequentcancellation of the main signal by virtue of those moving objectsregularly and frequently reflecting signals which lag the main signal byan odd number of half wavelengths.

[0171] In order to overcome this multipath problem should it arise inthe system 1 of the present invention, it is preferred that the antennasof the transmitters and receivers in each node 2 be highly directional.With a highly directional transmitter/receiver, there tends to be bettergain and therefore better signal strength than with an isotropicantenna. Thus, not only does a highly directional transmitter/receivertend naturally to detect only the main signal coming along theline-of-sight link 3 to the node 2 and does not detect reflected signalswhich approach the node at an angle to the main signal, a highlydirectional transmitter/receiver has improved operating characteristicsby virtue of the higher gain available. In addition to thehigh-directionality geometry of the antennas, circular polarisation ofthe transmitted radiation can be used to provide additional protectionagainst loss of signal due to multi-path effects. On being reflectedfrom a surface, a radio wave suffers a change in its phase relative tothe incoming wave. If this incoming wave is right-hand circularlypolarised, for example, then on reflection, this polarisation will bereversed to left-hand circular polarisation. In this way, unwantedreflected radiation is rejected relative to directly transmittedradiation if the receiver is selective to purely right-hand circularlypolarised radio waves. A similar argument would apply if left-handcircularly polarised receivers and transmitters were to be used. Thus,preferably, the system 1 uses line-of-sight, highly directional, highgain, high frequency transmitters/receivers equipped to emit and detectcircularly polarised radiation.

[0172] It will be appreciated that in the system 1 of the presentinvention, no base transmitter station is required and the system 1 canbe constructed from a single type of identical transceiver unit at eachnode 2. The network system 1 is potentially very much easier and cheaperto build, deploy and maintain in comparison with a cellular system whichuses base stations. There is no burying or suspending of cables or wiresor erecting of many large base-station antenna masts, again representinga large saving in costs and also minimising the environmental impact ofthe system 1. The capacity of the system 1 is very large as there aremany possible routes between nodes 2 and to the edge of the system 1.Failure of a particular node 2 accordingly implies loss of service foronly one user and other users are not normally affected as alternativepaths can be found for transmission of a signal. Because each node 2 isconcerned with switching as well as the transmission of informationtraffic, the whole system 1 effectively behaves as a distributed switch.This means that conventional access switches, which representsignificant capital expenditure, can be eliminated.

[0173] The present invention allows an operator to begin operating acommunications system 1 having very high data transfer rates to a smallnumber of users at relatively low cost. For example, 128 nodes can beset up in a system as described above at very low cost compared, forexample, to equivalent cable and cellular solutions. Subscribers to thesystem can be allocated respective nodes 2. The remaining nodes 2 whichhave not been allocated to a user can be used as “strategic” nodes 2solely for carrying information traffic between user nodes 2. As moreusers join the system, the strategic nodes can be allocated to the newusers. As the initially implemented system 1 fills so that all nodes 2are allocated to users, new nodes can be added and the system 1 as awhole can be reconfigured to introduce the new nodes to the system. Ifnecessary, a similar process in reverse can be applied to remove nodeswhich are no longer required or which are in maintenance or have failed,for example.

[0174] The maximum bandwidth which may be delivered to a node user fromthe network side (Bdown) and the maximum bandwidth a user may deliver tothe network (Bup) may be independently configured dynamically by thenetwork operator without affecting the capacity of the node for transittraffic. For example, a low tariff service might be Bup<<Bdown(similarly to ADSL service), whereas a higher-tariff service might allowBup=Bdown (‘symmetric’ service). Clearly both Bup and Bdown must be lessthan the peak user data rate allowed by the radio system.

[0175] A link 3 between two nodes 2 may actually comprise two or moreparallel radio channels, i.e. the link 3 uses simultaneously two or morefrequency channels, thus reducing the bandwidth load on a particularradio channel.

[0176] The overall control of routing of the signals between nodes canbe by virtue of a central controller. The central controller mightperform a periodic (e.g. daily) check on the system 1 as a whole todetermine whether any nodes 2 have failed. The central controller 1 canthen determine which route should be followed by a message from any onenode 2 to any other node 2 in the system 1. Appropriate instructionscould then be sent from the central controller to each node 2 so thateach node 2 applies an appropriate address to each information packet.

[0177] The present invention allows very high data transfer rates to beachieved. For example, as mentioned, a total node data transfer rate of40 Mbps is entirely feasible. Data transmission rates of 5 Mbps withburst rates of 25 Mbps can be achieved with ease.

[0178] An embodiment of the present invention has been described withparticular reference to the examples illustrated. However, it will beappreciated that variations and modifications may be made to theexamples described within the scope of the present invention.

1. A communications system, the system comprising: a plurality of nodes,each node having: receiving means for receiving via an antenna a signaltransmitted by wireless transmitting means; transmitting means forwireless transmission of a signal via an antenna; and, means fordetermining if a signal received by said node includes information foranother node and causing a signal including said information to betransmitted by said transmitting means to another node if said receivedsignal includes information for another node; each node having one ormore substantially unidirectional point-to-point wireless transmissionlinks, at least some of the nodes having plural substantiallyunidirectional point-to-point wireless transmission links which share atleast one of a receiver of the receiving means and a transmitter of thetransmitting means, each of said links being to one other node only, atleast some of the nodes being the origination and termination point ofuser traffic.
 2. A system according to claim 1, wherein the nodes arelinked so as to form transmission path loops thereby to provide pluralchoices of path for the transmission of a signal between at least someof the nodes.
 3. A system according to claim 2, wherein each loopconsists of an even number of links.
 4. A system according to any ofclaim 1, wherein for each node that has plural links to other nodes,each of said plural links to another node is associated with a timeslot.
 5. A system according to claim 4, wherein each link for each nodeis associated with a distinct time slot.
 6. A system according to claim4 or claim 5, wherein the allocation of time slots to the links can bevaried such that a link may selectively be associated with more than onetime slot.
 7. A system according to claim 1, wherein each node has adirect line-of-sight link with at least one other node such that eachnode can transmit a signal to another node in line-of-sight with saideach node.
 8. A system according to claim 1, wherein each node comprisesmeans for transmitting a signal including said information to anothernode if and only if a signal received at said node includes informationfor another node.
 9. A system according to claim 1, wherein each node isstationary.
 10. A system according to claim 1, wherein the number ofnodes is less than the number of links.
 11. A system according to claim1, wherein each node is arranged to be in a transmission mode for a timeperiod which alternates with a time period for a reception mode.
 12. Asystem according to claim 1, wherein at least one node is arranged notto transmit to any other node information in a signal received by saidat least one node when that information is addressed to said at leastone node.
 13. A system according to claim 12, wherein each node isarranged not to transmit to any other node information in a signalreceived by said at least one node when that information is addressed tosaid at least one node.
 14. A system according to claim 1, wherein eachnode has addressing means for adding to information in a received signalthe address of a node to which a signal including said information is tobe routed when said information is for another node.
 15. A systemaccording to claim 14, wherein the addressing means includes means fordetermining the route of information through the system and adding anappropriate address to the information accordingly.
 16. A systemaccording to claim 1, further comprising a central system controller fordetermining the route of information through the system.
 17. A systemaccording to claim 1, wherein at least one node has means fordetermining if a received signal includes information for said at leastone node and processing means for processing information in a signaladdressed to said at least one node.
 18. A system according to claim 1,wherein the transmitting means of the nodes are arranged to transmitsignals at frequencies greater than about 1 GHz.
 19. A system accordingto claim 1, wherein the link between two nodes is arranged to usesimultaneously two or more frequency channels.
 20. A system according toclaim 1, wherein said receiving and transmitting means are arranged totransmit and detect circularly polarised radiation.
 21. A systemaccording to claim 1, wherein the transmitting means includes a highlydirectional transmitter antenna.
 22. A system according to claim 1,wherein the receiving means includes a highly directional receiverantenna.
 23. A system according to claim 1, wherein each node issubstantially identical.
 24. A system according to claim 1, wherein thesystem is connected to a conventional trunk network for providing accessto other networks.
 25. A system according to claim 24, comprising afurther node connected by a data connection to one of the nodes of thesystem and arranged to transfer a signal to or receive a signal from thetrunk network or both.
 26. A system according to claim 25, wherein adata storage server is connected to or provided at a node.
 27. A systemaccording to claim 1, wherein at least one link of a node is arranged touse a first transmission frequency and at least one other link of saidnode is arranged to use a second transmission frequency.
 28. A systemaccording to claim 1, wherein some of the nodes are allocated tosubscribers and some of the nodes are not allocated to subscribers, atleast some of said non-allocated nodes being solely for carryinginformation traffic between subscriber nodes.
 29. A method ofcommunications across a network of nodes, each node having one or moresubstantially unidirectional point-to-point wireless transmission links,at least some of the nodes having plural substantially unidirectionalpoint-to-point wireless transmission links which share at least one of areceiver and a transmitter, each of said links being to one other nodeonly, the method comprising the steps of: (A) originating user data atone of the nodes; (B) transmitting a signal including said user datafrom said node to another node along a substantially unidirectionalpoint-to-point wireless transmission link between said nodes; (C)receiving said signal at said other node; (D) determining in said othernode if the signal received by said other node includes user data for afurther node and transmitting a signal including said user data fromsaid other node to a further node along a substantially unidirectionalpoint-to-point wireless transmission link between said nodes if saidreceived signal includes user data for a further node; and, (E)repeating steps (B) to (D) until said user data reaches its destinationnode.
 30. A method according to claim 29, wherein, for each node thathas plural links to other nodes, each of said plural links to anothernode is associated with a time slot, and each transmission step on alink of said one node occurs during a distinct time slot and eachreceiving step on a link of said other node occurs during a distincttime slot.
 31. A method according to claim 30, comprising the step ofvarying the allocation of time slots to the links such that a link isselectively associated with more than one time slot.
 32. A methodaccording to claim 29, wherein each node adds to user data in a receivedsignal the address of a node to which a signal including said user datais to be routed when said user data is for another node.
 33. A methodaccording to claim 29, wherein each node has addressing means, theaddressing means determining the route of the user data through thesystem and adding an appropriate address to the user data accordingly.34. A method according to claim 29, wherein a central system controllerdetermines the route of user data through the system.
 35. A methodaccording to claim 29, comprising the step of each node transmitting asignal including said user data to another node if and only if a signalreceived at said node includes user data for another node.
 36. A methodaccording to claim 29, including the steps of determining in at leastone node if a received signal includes user data for said at least onenode and processing the user data in a signal addressed to said at leastone node.
 37. A method according to claim 29, wherein the signals aretransmitted at frequencies greater than about 1 GHz.
 38. A methodaccording to claim 29, wherein there are at least two possible paths fortransfer of data between a source node and a destination node, andcomprising the step of transmitting a copy of said data on each of saidat least two paths.
 39. A method according to claim 29, wherein thereare at least two possible paths for transfer of data between a sourcenode and a destination node, and comprising the steps of transmittingfrom the source node a part only of said data on each of said at leasttwo paths and reconstructing the data from said transmitted parts ofsaid data in the destination node.
 40. A telecommunications switchingdevice, comprising a communications system according to claim 1.